1. Field of the Invention
The present invention relates to deployable structures. More particularly, the present deployable inflatable boom and methods for packaging and deploying a deployable inflatable boom provide controlled and predictable deployment behavior.
2. Description of the Related Art
Structures for use in space must be packaged, loaded into a rocket or shuttle, and launched into space. The cost of launching items into Earth orbit can exceed $10,000 per kilogram. Thus, any structure to be used in space is ideally as lightweight as possible. Further, rockets and shuttles have limited cargo space. Thus, many structures for use in space are designed to be collapsible so that they occupy little storage space relative to their deployed size. Once in space, the structures deploy to their usable configuration.
Inflatable structures offer the dual advantages of a compact packaged configuration and light weight, making them ideal for use in space. A cylindrical boom is a typical inflatable structure for use in space. Space booms are often used, for example, to support antennae and as masts for solar sails. The booms typically comprise elongated cylinders.
To package such a cylindrical inflatable boom for launch, the deflated boom is typically repeatedly folded lengthwise in an accordion fashion, such that when viewed in profile the folds in the boom trace a repeating Z. When the boom reaches orbit, pressurized gas aboard the spacecraft flows into the boom. As inflation proceeds, the boom unfolds.
Accordion folding, however, creates stress and wear along the creases in the boom. At each crease, a portion of the boom forms an outer layer and an adjacent portion of the boom forms an inner layer. The outer layer stretches as it is folded around the inner layer. The stretching weakens the boom at the creases, and makes the boom prone to fail during or after inflation. Boom failure typically prevents proper functioning of apparatus that was to be supported by the boom. Since space structures are often deployed aboard unmanned spacecraft, repairs to failed booms and malfunctioning apparatus are often impossible or too costly. Thus, failed booms can cause multi-million dollar spacecraft to be abandoned and left floating in space.
Stress and wear caused by accordion folding also complicates testing for booms. During testing, booms are typically packaged, deployed, deflated, and repackaged multiple times. The repeated folding and unfolding increases the stress and wear on the boom, thereby increasing the likelihood that the boom will fail during testing and prevent an accurate assessment of the boom""s capability to perform as desired.
Accordion folding also causes booms to deploy in an unpredictable fashion due to the effects of small inflation transients and friction between parts of the deploying boom. These effects can cause sudden changes in the deployment behavior of a boom or large gyrations in the movement of the deploying boom end. Preferably, however, a boom deploys in a predictable manner. The boom may damage or become caught upon neighboring structures if it does not deploy as desired.
During packaging of an inflatable boom, gas inside the boom is vented through the base of the boom to make the boom as small as possible. In space there is negligible pressure. Thus, even a miniscule amount of gas left inside the boom during packaging can produce high pressure within the boom after it is launched into space. With the accordion-fold method of packaging a boom, the vented gas must travel through a substantial length of the boom to be vented at the base of the boom. This substantial length coupled with friction between contacting inner surfaces of the boom causes small pockets of gas to collect near the folds of the packaged boom. This gas is never vented through the base of the boom. When the boom is launched into space, this residual gas further exacerbates the problem of uncontrolled and sudden partial deployment of the boom.
The accordion-fold method of packaging a boom is also not amenable for boom that to be used as a spar to support a membrane structure, such as a solar sail, a shade, or a reflecting surface. Multiple connectors along the boom""s length attach the membrane to the boom. Further, the membrane is folded and packaged with the boom. This packaging is very complex, and subjects the membrane to a high probability of damage during the folding and unfolding processes.
The preferred embodiments of the present boom and methods for packaging and deploying a boom have several features, no single one of which is solely responsible for their desirable attributes. Without limiting the scope of this boom and methods as expressed by the claims that follow, their more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled xe2x80x9cDetailed Description of the Preferred Embodiments,xe2x80x9d one will understand how the features of the preferred embodiments provide advantages, which include high packing density, controlled and predictable deployment, easy attachment of apparatus, such as a solar sail, high stiffness and strength, including high buckling strength, for minimum weight, easy packaging of apparatus together with the boom, sails can be attached to the boom using a following load that avoids high bending moments, capability to support very large structures, higher natural frequency than cylindrical boom of same radius, and minimum gas path length for more effective pre-launch venting.
A preferred embodiment of the present boom comprises an inflatable tapered cylindrical boom. The boom includes a substantially disk-shaped base, a membrane shaped substantially as a tapered cylinder, and a substantially cylindrical mandrel. A first wide end of the membrane is secured to the base, and a second narrow end of the membrane is secured to the mandrel.
Another preferred embodiment of the present boom comprises a method for packaging an inflatable tapered cylindrical boom. The method comprises the step of forming a plurality of folds in the boom, wherein each fold has a substantially circular shape when viewed from an end of the boom.
Another preferred embodiment of the present boom comprises a method for deploying an inflatable tapered cylindrical boom. The method comprises the step of increasing a gas pressure inside the boom, thereby unfolding an outer-most ring fold and elongating the boom.